1916] Packard: Mesozoic. and Cenozoic Mactrinae 305 



and prominent teeth," whereas his San Pablo specimen had a small 

 hinge with slender teeth. 



Remond described a form from the Miocene of Walnut Creek, 

 Contra Costa County, California, under the name of Cardium gabbii, 

 which later served as the type of Gabb's genus Pseudocardium. 

 Arnold, after making a study of the forms from the Coalinga Dis- 

 trict, California, came to the conclusion that Pseudocardium gabbii 

 and MuUnia densata were identical. He also described a form from 

 the Vaqueros under the name of M. densata var. minor. 



Another form was described from the Empire formation of Coos 

 Bay, Oregon, under the name of MuUnia oregonensis Dall. This is 

 based largely upon the rugose character of the shell and its narrow 

 shape. Both of these characters fall within the range of variation 

 of M. densata. 



Related forms were reported from the Miocene of Washington. 

 One was given specific rank while the others were listed as varieties 

 of Pseudocardium gabbii. The types of these four forms have 

 been studied and their relationships may now be more clearly shown. 



Specimens representing these different forms from a large num- 

 ber of localities have been studied from the standpoint of the gen- 

 eral shape and from that of the dental armature. The conclusions 

 of these studies are that these species and varieties as given above 

 in the synonymy are not separable into distinct species or even 

 varieties. The extremes in a suite from one collecting locality or 

 series made up of specimens from different localities and horizons 

 are often markedly different, but an attempt to so define the 

 extremes in terms that will serve to separate the middle members 

 has been unsuccessful. 



The figures show a few picked specimens of series showing the 

 relative prominence of the umbones, (plate 25), the general shapes, 

 (plate 26) and the characters of the dentition (plate 28). Plate 26 

 shows the amount of variation commonly found in the more ventri- 

 cose forms. This series represents specimens from a single collect- 

 ing locality, showing how the short tumid form grades into the 

 elongate flat type. Specimens that appear to have lived under 

 adverse conditions are shown on plate 27. The narrow, crude valves 

 of this series are not distinguishable from MuUnia oregonensis of 

 Dall, which is here considered synonymous with M. densata. 



Not only are there shape gradations between the typical and 



